Stationary gaseous-fueled internal combustion engines are commonly used throughout the world. These engines are typically powered by natural gas, methane, propane, or other suitable gaseous fuel. While these engines can be used in a multitude of applications, one typical use is in an agricultural setting having little or no access to electrical power at the growing fields. For example, powering irrigation pumps is a common use for these engines.
Emissions from these stationary gaseous-fueled internal combustion engines are a growing concern with stricter emission requirements being enacted for air pollution control. These engines may be periodically checked for compliance of the emission standards by an independent tester. One method of controlling the emissions of such engines is to control the air/fuel ratio of the engine.
It can be appreciated that air/fuel ratio controllers for stationary gaseous-fueled internal combustion engines have been in use for years. Some typical air/fuel ratio controllers are those described in U.S. Pat. No. 6,752,135 to Woodward Controls Corp. and U.S. Pat. No. 5,322,047 to Dynalco Controls Inc. While these devices may be suitable for the particular purpose to which they address, they are not as suitable for controlling the air/fuel ratio, i.e. lambda (refers to the amount of excess oxygen in the exhaust stream of an internal combustion engine) over a wide range of applications and not suitable to meet modern emission standards.
The system of U.S. Pat. No. 6,752,135 operates with a pressure regulator set to run at the lean limit and adding in a supplemental fuel flow through a small valve. The fuel added is modulated by a controller that uses an oxygen sensor in an attempt to maintain the correct mixture. The system has a limited range of operation and will not accommodate changes in the heating value of the gas as required by some applications. Since the system is already being run at the lean limit, if either the load is reduced dramatically or the heating value of the gas increases, the system will not be able to maintain compliance.
The system of U.S. Pat. No. 5,322,047 operates with a bias pressure applied to the back side of the pressure regulator, which is controlled by an electronic circuit and a current to pressure (I to P) transmitter. This system does not respond immediately when the load changes. It must wait until the oxygen sensor responds and also deal with the lag of the I to P transmitter.
These conventional air/fuel ratio controllers for stationary engines and others on the market typically use a heated exhaust gas oxygen (HEGO) sensor. As shown in FIG. 8, the HEGO sensor operates in only a very narrow range to switch between lean and rich, and thus only provides a rich or lean indication. For example, a HEGO sensor may operate with a 0 to 1 volt dc output with a stoichiometric air/fuel ratio at about 450 mV. The stoichiometric air/fuel ratio is the air/fuel ratio that is theoretically necessary for complete combustion. An engine's emissions are theoretically at their lowest at the stoichiometric air/fuel ratio, when used in conjunction with a catalytic converter.
For example, an output voltage of the HEGO sensor above 450 mV indicates a rich fuel mixture, and an output voltage of the HEGO sensor below 450 mV indicates a lean fuel mixture. However, small changes in the air/fuel ratio can cause the output voltage of the HEGO sensor to change abruptly above or below the 450 mV level. Plus, a difference of only 20 mV above or below 450 mV may be the difference between the engine running properly or not. Engine emissions are higher when the engine is run rich or lean.
Every time the HEGO sensor's output voltage transitions abruptly above or below this 450 mV level, the conventional air/fuel ratio controller responds by decreasing or increasing the amount of fuel provided to the engine. This controlling of the air/fuel ratio of the engine between rich and lean allows the controller to maintain a somewhat average air/fuel mixture, but the engine still exhibits increased emissions in these rich and lean conditions. While adequate in the past, this approach may not enable the engine to meet the latest emissions requirements for stationary gaseous-fueled internal combustion engines.
Also the HEGO may not operate satisfactorily in a lean burn application, limiting its usage. Another problem with conventional air/fuel ratio controllers are some systems may require the use of an outside source of pressure, either unregulated natural gas, which is very dangerous, or compressed air, which adds to the overall cost and complexity of the system. Still another problem with conventional air/fuel ratio controllers is that the software programs or human machine interface (HMI), may be very complicated and difficult to understand by all but the most astute in the art in at least some circumstances.